The proportion of outlay on quality assurance is gaining in importance given the continuously reducing pattern sizes on semiconductor products, as also in the field of flat panel displays. In general, for a product consisting of many planes, a measurement of test structures or component structures is carried out at least for each lithographically patterned plane, followed by comparison with desired or reference limiting values.
The corresponding measurements can be carried out with reference to absolute positional accuracy (registration), relative positional accuracy (overlay), layer thickness or pattern height, pattern width or pattern length, pattern edge angle, etc. An important subgroup of these measured data, which corresponds to a distance measurement between two points on the surface of a wafer slice, a mask, a flat panel display, etc., is denoted as CD (Critical Dimension) measurement. Because the optical resolution limit is being reached due to the continuously reducing pattern sizes, increasing use is also being made of scanning electron microscopes (SEMs) instead of photooptical measuring instruments. Such a CD measurement for determining the characteristic dimensions, for example, of predefined patterns such as lines, rectangles, slits, etc., can validate the quality of a preceding lithographic step.
A series of aligning and measuring steps are generally carried out in order to perform the CD measurement in an optical or scanning electron microscope. A sequence of such measuring steps is illustrated schematically, for example in FIG. 1. After lithographic patterning has been carried out in a lithography cluster including resist coating, hot and cool plates, exposing the resist, cooling down, developing and hardening, as well as cleaning, etc. The first step, in this case, is to carry out an overlay measurement to check the accuracy of the alignment of the currently patterned plane with reference to preceding planes. Such measurements are usually accomplished in measuring instruments specifically set up therefore.
After the overlay measurement, the product to be measured, for example, a semiconductor wafer, is transferred to the CD measuring instrument, in which the semiconductor wafer is aligned on a stage, i.e., a substrate holder. A first measuring step includes a global alignment. In this measuring step, alignment marks set up specifically for this purpose are used to align the stage with the wafer into a defined co-ordinate position relative to the optical or electronic lens system.
The stage with the wafer is positioned by positional data taken from the pattern design such that the structure to be measured passes roughly into the raster image field of the lens system. With digital image processing, the targeted pattern is detected in this image field by pattern recognition methods, and the semiconductor wafer is subsequently readjusted.
In a further step, the lens system is aligned such that as high as possible a resolution or sharp definition is achieved for the imaging. Like the step of pattern recognition, this step, known as autofocus, is sufficiently well known from the prior art to the average person skilled in the art. Autofocus steps can be implemented in the case of optical microscopes by varying lens separations, and in the case of scanning electron microscopes, by varied current intensities or induction intensities in the lens coils.
A further measuring step, which relates chiefly to scanning electron microscopes, is the checking of the stigmation quality, i.e., the astigmatism. In this case, the setting values, determined from a first measuring curve in a first direction, for example, “X”, for the focus are compared with those of a second one, in a second direction, for example, “Y”, orthogonal to the first direction. Depending on a result to be achieved, which is a function of the pattern design to be imaged, the lens system can be readjusted here once more.
Only after this step does the actual CD measurement take place, for example, by selecting two opposite edge points for the purpose of measuring the width of a pattern as characteristic dimension, and measuring their spacing.
The measured value of the pattern width is then fixed as CD value for this pattern and, if appropriate, compared, by averaging together with further pattern width measurements, to a tolerance band, prescribed from the design rule, for CD values. When the limits of this tolerance band are exceeded, the semiconductor wafer must be passed on for reworking in the normal case after the lithography step.
The subsequent process step, in this case, an etching step, can be carried out when the tolerance limits are observed.
Whereas in past years the measuring accuracy of the scanning electron microscopes has sufficed completely for distinguishing different dimensions of patterns within a prescribed tolerance band, this is becoming more difficult nowadays with the continuously decreasing pattern sizes. For example, a typical 10% tolerance band of a 150 nm wide gate stack of a transistor, for example, of a memory product, is 15 nm. Consequently, an accuracy of 3 nm is to be achieved by the scanning electron microscope given the requirement of a 20% resolution within this tolerance.